Printer problem: not receiving response from adapter

I'm running HP-UX 10.20 and print to a "TCP/IP" networked printer, a Lexmark Optra Rt+. I have Lexmark's Markvision software, version 3.4.

My NT PCs also print to this printer.

Intermittently, when I print a job from UNIX (using lp) I discover the UNIX printer queue becomes disabled i.e.
lpstat -p<printer queue>

printer <printer queue> disabled ...
disabled by scheduler

(The printer queue is enabled before I do the lp.)

I also see in Markvision's printer log this error:
Not receiving response from adapter

I do not see an error message in a "lp" log.

I also encounter this problem on other Lexmark printers (e.g. Optra S 2450).

A couple of times when I've "noticed" the printer queue disabled I've gone to the printer:
- once, it was in the process of printing a very large "PC" job
- another time, it was printing a "PC" job and had run out of paper

Does anyone have any ideas? So far Lexmark hasn't been much help.

Thanks, Lynda